Fuel cells convert the chemical energy of fuel into electrical energy. Examples of fuel cells are polymer electrolyte fuel cells (“PEFCs”) and alkaline fuel cells (“AFCs”). PEFCs have a relatively simple cell design and use a gaseous fuel (e.g. hydrogen, dimethyl ether and the like) or liquid fuel (e.g. methanol, ethanol, ethylene glycol, glycerol, hydrazine, and the like) that can easily be delivered to the cell and has a high energy density. PEFCs require expensive precious metals (e.g. platinum) as electrocatalysts, and operate under acidic conditions, for which the reduction of oxygen and/or the oxidation of the liquid fuel is slow. By contrast, AFCs may use relatively inexpensive catalysts made from base metals (i.e. non-precious metals) that tend to have a high activity under alkaline conditions.
A schematic diagram of a solution-based, H2/air AFC is shown in FIG. 1. The half reactions at the anode and cathode are:3H2+6OH−→6H2O+6e−  (anode)3/2O2+3H2O+6e−→6OH−  (cathode)As FIG. 1 shows, oxygen passes through the cathode gas diffusion layer (GDL) to the cathode where it reacts with water and electrodes to form hydroxide ions (OH−). The hydroxide ions generated at the cathode are transported through an anion exchange membrane to the anode. Hydrogen fuel passes through the anode gas diffusion layer where it reacts with the hydroxide to generate water and electrons. Electrons flow through an external circuit from the anode to the cathode.
A solid alkaline fuel cell (“SAFC”) is a type of AFC that has a solid electrolyte. Solid alkaline fuel cells have certain advantages compared to liquid electrolytes. For example, solid electrolytes occupy a smaller volume than liquid electrolytes do, and solid electrolytes are also less corrosive than liquid electrolytes are.
Currently, the performance and durability of solid alkaline fuel cells (“SAFCs”) is inferior to that for polymer electrolyte fuel cells (“PEFCs”). This is due, at least in part, to the conductivity, mechanical properties, and degradation of the polymeric materials used in the membranes and electrodes of SAFCs. PEFCs use solid electrolytes that are cation exchange polymer membranes while SAFCs use solid electrolytes that are anion exchange polymer membranes. Anion exchange membranes tend to exhibit lower ion conductivities than the cation exchange membranes. Anion exchange membranes also tend to have poorer mechanical properties than the cation exchange membranes have. Anion exchange membranes also tend to degrade faster under fuel cell operating conditions than the cation exchange polymer membranes do. There are also problems associated with the electrodes for SAFCs. Electrode reactions for SAFCs occur at the interface between electro catalysts and polymer electrolytes. A significant loss in fuel cell performance is due to, i) degradation of cation functional groups under high pH conditions and ii) strong cation absorption on the surface of the electrocatalyst. Despite the known problems associated with alkaline exchange polymers, renewed interest has grown in the development of alkaline fuel cells in recent years because the efficiency of the oxygen reduction reaction of electrocatalysts in alkaline environment is likely greater than in an acidic environment and therefore, expensive platinum based catalysts can be replaced with inexpensive ones such as nickel, silver and carbon [1, 2, 3].
Quaternary ammonium-tethered anion exchange polymer electrolytes [4, 5, 6, 7, 8, 9] have been the most extensively studied. Degradation of alkyl ammonium-based polymer electrolytes occurs under high pH conditions via i) Hoffmann elimination (E2) ii) nucleophilic substitution (SN2) [10] or iii) ylide formation [11]. Although the E2 reaction can be circumvented by avoiding a coplanar arrangement of β-hydrogen and nitrogen or by synthesizing β-hydrogen-absent quaternary ammoniums, most of the alkyl ammonium functionalized polymers still have not realized sufficient stability due to the SN2 reaction. In the SN2 reaction, hydroxide ions attack the α-carbon of the ammonium cations [12]. The extent of polymer degradation via the SN2 reaction may be reduced by replacing alkyl ammonium with bulky cations such as guanidinium or phosphonium, which stabilize the α-carbon-nitrogen bond by charge delocalization of the cations [11]. Degradation through the ylide pathway starts with hydroxide ion attack on a CH3 proton and produces a water molecule along with an ylide intermediate (N(CH3)3+CH2−). In addition, quaternized ammonium tethered to anion exchange polymer electrolytes have adhered strongly on the surface of electro-catalyst and lower the efficiency of electro-chemical reactions.
Most cation-functionalized polymer electrolytes have been prepared via chlorination, or bromination, of methyl groups followed by cationization of the resulting halomethyl group [7, 8, 13, 14]. Benzyl guanidiniums, however, degrade via the nucleophilic attack by hydroxide ion at the benzylic cation [15]. In addition, the degradation of benzyl ammonium through ylide formation and subsequent rearrangement reactions may limit the lifetime of the polymers [16].